This is not my area of law or expertise, but I believe that the federal government demands that "X" amount of hours of broadcasting be broadcast "in the public interest" if these companies are to retain their broadcast license. I don't know if "news" is specifically mentioned in statute, or in case law, but I think the broadcast networks have at least some legal obligation to broadcast a "news" program".
This was reinforced at some point - again, I think - in the late 60's when the FCC gave the networks some additional time to broadcast commercials during each hour. The networks agreed to children's programming and news or other public interest programming requirements for that extra time.
Having said that, NBC still makes a TON of money off news, and ABC/CBS still make money, not just the kind of money they'd like. It's not a "loser" , yet.
You may be right but there it seems that every TV market has one or two re-run channels that never seem to show news and they have broadcast licenses. Also, is it the network that has a FCC license or the individual stations? No sure, but yours is a good point.